The invention relates to a photoelectric conversion circuit, and more particularly, to such a circuit for use in a camera or the like for photometry of light from an object being photographed and for converting it into a corresponding electrical signal.
In photographic cameras and illumination photometers, the photometry takes place by determining extraneous light such as light from an object being photographed, deriving an electrical signal which is proportional in magnitude to the amount of light and using the electrical signal to control an exposure operation or a display on the photometer. A photoelectric conversion circuit is used to convert a light signal into an electrical signal, which is processed in terms of an analog quantity. For a camera, for example, a variety of photographing information including film speed must be supplied to the camera, which also must be processed in an analog manner. This disadvantageously results in a very complex arrangement. A technique has been proposed which converts an analog signal into a corresponding digital signal for digital processing purpose. The digital processing simplifies the processing when introducing a variety of photographing information into a photographic camera, for example, and also improves the accuracy of the photometry. When the digital processing technique is employed, the analog signal is once converted into a digital signal before it is processed. Subsequently, the digital signal must be converted into an analog signal again in order to control the pointer of an exposure meter, for example. However, the use of the analog-to-digital conversion is advantageous when the overall merits are considered.
However, a conventional digital processing technique in a photographic camera, for example, comprises compressing a photocurrent derived from a silicon photodiode, for example, by a logarithmic comparison circuit, and then converting the compressed signal into a digital signal in an analog-digital conversion circuit. The logarithmic compression circuit requires the provision of a temperature compensation circuit and a constant voltage circuit. A variety of techniques are employed to form an analog-digital conversion circuit, including a sequential comparison, a count comparison, a double integration, a follow-up comparison type. However, a conventional digital processing circuit is very complex in its entire arrangement and requires an increased power dissipation. Since such circuits are subject to temperature changes and fluctuations in the supply voltage, there must be provided a temperature compensation circuit and a voltage fluctuation compensation circuit, adding to the complexity of the entire arrangement. In addition, the conventional circuit arrangement suffers from disadvantages that it cannot respond immediately after the power supply is turned on and that it cannot respond sufficiently rapidly.